The present invention relates to improvements in or relating to cooling apparatus, and is more particularly, although not exclusively, concerned with cooling apparatus for magnets having improved temperature stability for use in a magnetic resonance imaging (MRI) systems.
MRI systems incorporate magnets to generate a static magnetic field, with a high degree of homogeneity across a volume of interest. Homogeneity of 3 parts per million (ppm) rms across a 40 cm diameter sphere are typically achieved. Moreover, the magnetic field requires a high degree of stability with respect to time in order to enable imaging within a reduced time interval.
The majority of known MRI systems consists of a plurality of solenoidal superconducting coils which are coaxially arranged inside a container filled with liquid helium. This helium is evaporating at atmospheric pressure, which results in high temperature stability, which in turn ensures high dimensional stability of the magnet. This results in a magnetic field with the required stability.
Superconducting solenoidal magnets produce high quality fields in an efficient way in terms of field energy across the imaging volume per unit cost. However, a major drawback of solenoidal systems is the lack of openness and access. This lack of openness excludes the imaging of patients who suffer from claustrophobia and/or of patients connected to peripheral medial equipment, or those who, due to their medical condition, do not fit inside the cylindrical bore of a conventional solenoid magnet.
In recent years, magnet systems have been proposed with much improved accessibility and openness. GB patent application No. 9925513.5 discloses an improved magnet which, when used in an MRI system, allows greater access to the patient and is more open, thereby reducing the likelihood of claustrophobia. The majority of these open magnets comprise a significant amount of ferromagnetic material, such as low carbon steel in the magnet yoke, to guide the field or to control the homogenity of the magnetic field. A number of these magnets also use permanent magnetic material, such as NdFeB, SmCo or ferrite, to generate the magnetic field. Note that ferromagnetic materials are also used in the aforementioned solenoidal systems to correct small deviations from the specified field due to manufacturing tolerances or environmental factors.
The magnetisation of these ferromagnetic materials is temperature dependent. Where these ferromagnetic materials contribute to the central field strength, temperature deviations will result in essential proportional changes in the magnetic field. If the ferromagnetic structure also contribute to higher order zonal harmonics, the change in temperature will also affect the diameter of the sphere in which the peak to peak homogeneity value of the field is less than 3 ppm. This is important for the imaging of fatty tissue.
Changes in temperature of the magnet structure can be due to changes in ambient temperature, variations of temperature in the gradient coils and the rapid changing magnetic fields of the gradient coils (eddy currents).
Stabilisation of the temperature of the ferromagnetic structures and/or the permanent magnetic structures is an obvious way to counter the effects of these temperature variations and dissipation due to eddy currents.
GB-A-2 341 449, GB-A-2 341 448 and GB-A-2 341 447 (publications of GB patent applications No. 9819740.3, 9819724.7 and 9819694.2 respectively) disclose temperature control means for a MRI magnet which consist of an electronic system comprising temperature sensors, electronic heaters/coolers and a controller.
However, such a temperature control system requires the use of relatively expensive and complex components.
It is therefore an object of the present invention to provide a temperature control system which is relatively inexpensive to implement.
It is another object of the present invention to provide a temperature control system which utilises simple components.
It is a further object of the present invention to provide a magnet arrangement with improved temperature stability for use in MRI apparatus in which a significant contribution of the magnetic field is from magnetised material.
In accordance with one aspect of the present invention, there is provided a temperature control system including:
a substance having a phase transition temperature at which it changes from a first state to a second state;
means for maintaining the substance at its phase transition temperature substantially in the first state;
sensor means associated with the substance for providing an output signal indicative of the state of the substance; and
control means for receiving the output signal to control the means for maintaining the substance at its phase transition temperature substantially in the first state.
In one embodiment of the invention, the means for maintaining the substance at its phase transition temperature substantially in the first state includes a heating element. A thermal sink may also be provided for removing heat from the substance. Optionally, a thermal resistance may be located between the substance and the thermal sink.
In another embodiment of the present invention, the means for maintaining the substance at its phase transition temperature substantially in the first state includes a Peltier device.
Advantageously, the Peltier device operates as both a heating element and a heat pump to provide heat to and to remove heat from the substance.
The substance may comprise a wax having a first state which is substantially solid and a second state which is substantially liquid. In this case, the sensor means may comprise an optical arrangement for detecting optical changes from the first state to the second state. It is preferred that the optical arrangement comprises an optical emitter and an optical receiver immersed in the wax, the optical emitter being a light emitting diode and the optical receiver being a photodiode.
Additionally, a liquid substance having a different density and phase transition temperature may form a convection layer for the substance. The liquid substance may form a convection layer either over or under the substance.
The substance may comprise a cooling liquid having a first state which is substantially liquid and a second state which is substantially gaseous. In this case, the sensor means comprises a pressure sensor connected to detect pressure changes as the cooling liquid changes from the first state to the second state. Additionally, a condensing chamber is provided for condensing the second state of the cooling liquid, the pressure sensor being connected to detect pressure changes in the condensing chamber.
In accordance with another aspect of the present invention, there is provided magnetic resonance imaging apparatus including a plurality of magnetic elements and a temperature control system as described above for cooling the magnetic elements, the substance being in direct contact with the magnetic elements.